Acrylic acid is useful as a binder, and acrylic acid is an important chemical used for the use such as a monomer for preparing a polymer that is used by being dispersed in an aqueous medium.
Acrylic acid is mainly prepared in a manner in which propylene is primarily oxidized to acrolein, which is then subsequently oxidized.
Specifically, FIG. 1 shows the process of producing acrylic acid (AA) from propylene. As shown in this drawing, propylene is reacted with oxygen in air using a Mo—Bi-based first-step catalyst and a Mo—W-based second-step catalyst and is thus almost completely consumed (conversion of 97% or more). Among reaction products, a condensable oxygenated product including AA is condensed into a liquid phase in an absorption tower and then transferred to an AA separation tower. About 30% of a stream, including not only unreacted propylene but also nitrogen (N2), oxygen (O2), carbon monoxide (CO), carbon dioxide (CO2) and water (H2O), is recycled. The recycled stream contains a large amount of nitrogen (N2), about 25 to 30 vol % of water (H2O), and small amounts of oxygen (O2), carbon monoxide (CO) and carbon dioxide (CO2). The recycled stream is mixed with propylene and air at the inlet of a reactor and is then introduced to the reactor, and about 70% of the remainder thereof is purged and then incinerated in an incinerator. The stream supplied into the reactor, together with the recycled stream, has the following composition at the reactor inlet: 7% propylene, 12.6% oxygen (O2) (from air), 72.4% nitrogen (N2) (47.4% from air and 25% from the recycled stream), and 8% water (H2O) (from the recycled stream).
A propane-based acrylic acid production process (POA) for preparing acrylic acid from propane has not yet been commercialized because of low propane conversion and acrylic acid selectivity. FIG. 2 schematically shows a propane-based acrylic acid production process, to which a propylene-based acrylic acid production process is applied without change. This method suffers from excessive loss of unreacted propane. For example, U.S. Pat. No. 7,304,014 discloses a propane conversion of 68% and an acrylic acid selectivity of 80% through a single pass, but is problematic in that when the propane-based acrylic acid production process is applied to the propylene-based acrylic acid production process, only 30% of the 32% unreacted propane fraction is recycled, and the remaining 70% thereof is purged in an incinerator, undesirably resulting in propane loss of 22.4% (0.32*70).
A catalyst mainly used in the propane-based process is MoVTeNb. In the presence of a MoVTeNb-based catalyst, when the propane/O2 ratio is high, propane conversion is lowered. On the other hand, when the propane/O2 ratio is low, the propane conversion is increased, but COx selectivity, such as CO, CO2, etc. is increased, that is, the selectivity of oxygenated carbon, is increased, thus decreasing an acrylic acid yield or an acrylic acid selectivity.
A currently available propane-based acrylic acid production process is problematic because of the large amounts of unreacted propane and non-condensable gas, such as CO, CO2 or propylene, undesirably causing excessive energy expense upon separation of byproducts and requiring an additional unit for removing CO and CO2.